While through efforts like the Surviving Sepsis Campaign and others, supportive therapies have improved survival of the critically ill septic patient, nonetheless, a substantial number still develop this morbid syndrome and die. Unfortunately, there presently exists no true medicinal/ molecular therapeutic agent that can be applied to treat sepsis or diagnosis/ prognosis (beyond Pro-calcitonin) the trajectory of the critically ill patient. Thus, the need remains to further clarify the complex pathobiology of the septic process. In this respect, our laboratory has had sustained interest in understanding how sepsis, as produced by cecal ligation and puncture (CLP), differentially affects the immune response observed in divergent tissue sites. Importantly, we have observed that CLP induces marked changes in the functional responsiveness of monocyte/ macrophages, as well as in key regulatory T-cell populations, i.e., invariant natural killer T (iNKT)-cells and CD4+CD25+ native T- regulatory (T-reg)-cells (within the spleen, liver and intestine), which we have found to either have the ability to induce marked immune suppression or have an effect on the animal's capacity to ward off septic morbidity/ mortality. Most significantly, we have recently shown that a novel family of co-inhibitory molecules/ receptors as well as their cell-associated ligands, belonging to the Programmed Cell Death Receptor family, appear to be playing a unique and very proximal role in both immune cell/ organ morbidity and mortality seen not only in experimental septic mice, but their increased expression is associated with poor outcome in critically ill patients. With this in mind, we propose the following hypothesis that classic adaptie/ lymphoid co- inhibitory receptors, such as programmed cell death receptor (PD)-1 and B & T lymphocyte attenuator (BTLA) and/or their ligands (PD-L1 or PD-L2 for PD-1 & HVEM for BTLA), contribute to the development of septic morbidity via their novel myeloid cell and/or select regulatory lymphoid subset interactions with other immune/ non-immune (endothelial and/or epithelial) cells present in a given tissue. In AIM 1: We will establish the extent to, whic macrophages, neutrophils and regulatory lymphoid cell's expression of PD-1 and/or BTLA contributes to the development of immune cell dysfunction in sepsis. AIM 2: We will determine how and what non-immune cells express the ligands for PD-1 or BTLA, i.e., PD-L1, PD-L2 and/or HVEM, respectively, following the onset of sepsis; how this contributes to the development of organ dysfunction and/or subsequent survival of sepsis; and what the significance of this expression is to liver or intestinal endothelial and/or epithelial cell dysfuntion seen in sepsis. We firmly believe the results of these studies will provide information that not only allows us to better understand the pathobiology of sepsis both experimentally in mice and critically ill patients, but also point to agents that can be used to possibly diagnose, prognosticate and/or therapeutically inhibit the progression of sepsis.